Image sensors utilize an array of photo-sensitive pixels to capture an optical image. One common type of image sensor is a Complementary Metal Oxide Semiconductor (CMOS) image sensor. There are a variety of techniques to package CMOS image sensors. However, one technique that is used is to mount the image sensor on a substrate (interposer) and to provide wire bond connections between the substrate and the image sensor. An additional transparent cover is often provided to protect the image sensor.
Reliability of the packaged image sensor is typically increased if the packaged assembly is encapsulated. One approach that is used in the prior art is to use an encapsulant (e.g., a liquid polymer) to simultaneously encapsulate the wire bonds and provide a moisture seal about the edges of the transparent cover. For example, FIG. 1 illustrates a packaged assembly 50 described in U.S. Pat. No. 6,995,462 having an interposer 30 (with external connection points 38 and solder balls 48), image sensor chip 4, transparent cover 22, wire bonds 44, and attachment pads 34. An encapsulant 46 (e.g., a silicon-filled polymer or liquid crystal polymer) encapsulates the wire bonds 44. In this package design a micro-lens 20 is disposed over a portion of the image sensor 4 that has an array of pixels. A liquid adhesive material 24 (such as a liquid epoxy or a liquid resin) is used to attach the transparent cover. However this structure has several drawbacks. Conventional liquid adhesives are difficult to control in regards to thickness and width. Thus, the width of the liquid adhesive is greater than desired, wasting space on the die and thereby increasing manufacturing costs. Additionally, only a comparatively small amount of encapsulant exists at the edges of the transparent cover. That is, at the edges of the transparent cover the liquid epoxy borders the encapsulant such that only a comparatively thin region of encapsulant provides moisture protection at the edge of the transparent cover.
FIG. 2 illustrates a film over wire structure that is a variation of the structure described in U.S. Pat. No. 6,388,313, assigned to Siliconware Precision Industries Co. Ltd (SPIL) of Taiwan. The SPIL CMOS image sensor package assembly includes an interposer to which an image sensor chip is mounted. A liquid adhesive film, such as a liquid resin film, is used to bond the transparent glass cover. However, it is difficult to accurately control the spreading of a large area liquid resin film due to resin bleed. That is, in addition to other drawbacks a film made of a liquid resin will tend to flow laterally across the edge of the die after an initial film deposition. As a result, this means that it is difficult to achieve a narrow gap between the pixel array edge and the die edge. This wastes die area which, in turn, results in greater manufacturing costs than desired. Additionally, moisture permeation of a liquid adhesive resin film is a reliability concern. In particular, there is a risk that moisture can permeate laterally through the resin film during the lifetime of the packaged image sensor.
Therefore in light of the above described problems an improved apparatus, system, and method to package an image sensor was developed.